This test shows a worst case scenario which emphasizes suspension and motor distortion and the effectiveness of feedback to clean it up This speaker produces loads of distortion below it's resonant frequency. The lower the frequency the more dominant the distortion mechanisms. I would not recommend it for HiFi use

The following measurements compare the effectiveness of current drive alone in reducing high frequency distortion. About a 14 dB reduction in distortion can be achieved with current drive depending on the drive level. Unfortunately current drive is worse than voltage drive at frequencies below the resonant frequency which is where the global feedback is effective

Impressive!
Will for sure be interested in buying one of these when you finish!
5 years ago I made an analogue servobass system with the same accelerometer and was able to drop THD from 26% to 5% on my Peerless XLS 10" woofer. This was at full X-MAX of 12mm and at 23Hz. I ran into problems with the soft dust-cap that created resonances at about 500Hz. I had to mount the accelerometer directly over the voice coil which again created wrong balance in the cone. The accelerometer weighs 4 grams and when the g-forces reached about 70g the coil crashed into the magnet
Anyway, it seems your system has much better potential. Have you looked into mounting one of these super-small chip accel availiable? http://www.analog.com/static/importe...ts/ADXL193.pdf

Can you show me the open loop in a bode plot? It looks nice, but the DSP*Wooferdynamics is what is the really interesting part.

Ideally a separate DSP implementation and a separate Woofer-model.

Next to that, have you measured the latency of the DSP from analog in to analog out?

A lot of it is proprietary so can't share too many details, suffice to say that I use a novel approach that allows me to define the frequency response or speaker alignment independent of the servo amplifier response. Most servo designs have to build the closed loop frequency response around the servo amplifier and the requirements of stability. Currently I use a simple 2nd order high pass shaping filter and I can select the cutoff frequency and Q independent of the servo amplifier bandwidth and gain. You could use other shaping filters such as 3rd or 4th order Butterworth or Bessel etc which would give additional subsonic filtering but poorer group delay and transient response.

Regarding latency the total group delay or transit time of the adc and dac chain is 7.5/fs @192KHz. This is about 37.5uS. But add to that the extra dsp processing and delay in the analog circuitry gives about a net 4 degree phase shift at 100Hz. To put that into perspective the phase shift of the compensation of the servo amplifier swamps the group delay by a large margin whilst still maintaining unconditional stability

Impressive!
Will for sure be interested in buying one of these when you finish!
5 years ago I made an analogue servobass system with the same accelerometer and was able to drop THD from 26% to 5% on my Peerless XLS 10" woofer. This was at full X-MAX of 12mm and at 23Hz. I ran into problems with the soft dust-cap that created resonances at about 500Hz. I had to mount the accelerometer directly over the voice coil which again created wrong balance in the cone. The accelerometer weighs 4 grams and when the g-forces reached about 70g the coil crashed into the magnet
Anyway, it seems your system has much better potential. Have you looked into mounting one of these super-small chip accel availiable? http://www.analog.com/static/importe...ts/ADXL193.pdf

Yes I bought some NOS ADXL250's some time ago from ebay but didn't use them in the end. The problem with some of the mems devices is they have much more noise compared to the piezo devices. The ACH-01-03 is a very low noise device which makes it ideal for this type of app.

There are even better Measurement Specialities devices I have looked at such as the 805 and 805M1 but they are more expensive and need to be mounted on a pcb etc and don't have an integral flexible coax lead.

Mounting of the accelerometer is crticial. As soon as you mount it on a dust cap you create a mechanical resonance depending on the weight of the accelerometer and the mechanical compliance of the dust cap. This resonance can vary from a few hundred Hz to the KHz region and makes this type of mounting totally futile in most cases. The end result is to make the system unstable and turn it into an oscillator.

I have an 8 inch aluminum coned woofer and I mounted the accelerometer onto the aluminum dustcap using bluetack of all things. It works quite well but exhibits a sharp peak resonance of 15 dB at 1.3KHz. Without filtering the system will oscillate at about 1.3KHz. But I have built in a user defined fully customizable anti-modal filter which can essentially null out this resonance and eliminate any instability At the end of the day the accelerometer should be tightly coupled to the voice coil but even then you have to deal with modal resonances in the speaker cone. Again the anti-modal filter built into the dsp software comes to the rescue